The present invention relates to multi-color electrophoretic media and to displays incorporating such media.
Traditionally, electronic displays such as liquid crystal displays have been made by sandwiching an optoelectrically active material between two pieces of glass. In many cases each piece of glass has an etched, clear electrode structure formed using indium tin oxide. A first electrode structure controls all the segments of the display that may be addressed, that is, changed from one visual state to another. A second electrode, sometimes called a counter electrode, addresses all display segments as one large electrode, and is generally designed not to overlap any of the rear electrode wire connections that are not desired in the final image. Alternatively, the second electrode is also patterned to control specific segments of the displays. In these displays, unaddressed areas of the display have a defined appearance.
Electrophoretic display media, generally characterized by the movement of particles through an applied electric field, are highly reflective, can be made bistable, and consume very little power. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Encapsulated electrophoretic displays also enable the display to be printed. These properties allow encapsulated electrophoretic display media to be used in many applications for which traditional electronic displays are not suitable, such as flexible displays. The electro-optical properties of encapsulated displays allow, and in some cases require, novel schemes or configurations to be used to address the displays. Accordingly, such displays have been the subject of intense research and development for a number of years.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Nevertheless, problems with the long-term image quality of non-encapsulated electrophoretic displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in the these patents and applications include:                (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. No. 7,002,728; and U.S. Patent Application Publication No. 2007/0146310;        (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276 and; 7,411,719;        (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. No. 6,982,178; and U.S. Patent Application Publication No. 2007/0109219;        (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. No. 7,116,318; and U.S. Patent Application Publication No. 2007/0035808;        (e) Color formation and color adjustment; see for example U.S. Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; and 7,167,155; and U.S. Patent Applications Publication Nos. 2004/0190114; 2004/0263947; 2007/0109219; 2007/0223079; 2008/0023332; 2008/0043318; and 2008/0048970;        (f) Methods for driving displays; see for example U.S. Pat. No. 7,012,600; and U.S. Patent Application Publication No. 2006/0262060;        (g) Applications of displays; see for example U.S. Pat. No. 7,312,784; and U.S. Patent Application Publication No. 2006/0279527; and        (h) Non-electrophoretic displays, as described in U.S. Pat. Nos. 6,241,921; 6,950,220; and 7,420,549.        
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Electrophoretic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.
As already indicated, an encapsulated or microcell electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Most prior art electrophoretic media essentially display only two colors. Such electrophoretic media either use a single type of electrophoretic particle having a first color in a colored fluid having a second, different color (in which case, the first color is displayed when the particles lie adjacent the viewing surface of the display and the second color is displayed when the particles are spaced from the viewing surface), or first and second types of electrophoretic particles having differing first and second colors in an uncolored fluid (in which case, the first color is displayed when the first type of particles lie adjacent the viewing surface of the display and the second color is displayed when the second type of particles lie adjacent the viewing surface). Typically the two colors are black and white. If a full color display is desired, a color filter array may be disposed over the viewing surface of the monochrome (black and white) display. Such a color filter array is typically of the red/green/blue (“RGB”) or red/green/blue/white (“RGBW”) type. Displays with color filters rely upon an area sharing approach with three (in the case of RGB displays) or four (in the case of RGBW displays) sub-pixels together functioning as a single full color pixel. Unfortunately, each color can only be displayed by part of the display area. For example, in an RGBW display, each of red, green and blue can only be displayed by ¼ of the display area (one sub-pixel out of four) and white can effectively be displayed by ½ of the display area (one complete sub-pixel out of four, plus each colored sub-pixel acts as ⅓ white, so the three colored sub-pixels together provide another one complete white sub-pixel). This area sharing approach result in colors less bright than is desirable.
Alternatively full color displays can be constructed using multiple color-changing layers, with at least one front (i.e., adjacent the viewing surface) color-changing layer operating in shutter mode. Apart from being complicated and potentially expensive, such a multi-layer display requires precise alignment of the various layers, and highly light transmissive electrodes (and transistors, in the case of an active matrix display).
The aforementioned U.S. Pat. No. 6,017,584 describes an electrophoretic medium having three different types of particles having three different colors in a colored or uncolored fluid, and a method of driving the particles so as to enable each of the three different colors to be displayed. The relevant disclosure from the aforementioned U.S. Pat. No. 6,017,584 is reproduced below with reference to FIGS. 6-9 of the accompanying drawings.
There is still, however, a need for electrophoretic media capable of displaying more colors at each pixel in order that, for example, such media can reproduce the appearance of high quality color printing. Such high quality printing is typically effected using at least four inks, cyan/magenta/yellow/black (“CMYK”). It is often not appreciated that a so-called “four-color” CMYK printing system is in reality a five-color system, the fifth color being the white background provided by the paper (or similar) surface when no ink is applied thereto. Since there is no comparable background color in an essentially opaque electrophoretic medium unless it is being used in shutter mode, a non-shutter mode electrophoretic medium should be capable of displaying five colors (black, white and three primary colors, the three primary colors typically being cyan, magenta and yellow. It has now been realized that by this aim can be achieved by using the electrophoretic medium from the aforementioned U.S. Pat. No. 6,017,584 having three different types of particles in a colored fluid and choosing the colors of both the particles and the fluid carefully.